Wood and biomass burning is making a comeback after over a century of domination by coal, petroleum, and natural gas for power generation. The availability of energy-dense fossil fuels and efficient transportation networks made centralized power production the technology of choice. In the 21st century, biomass heat and power plants and district heating are enjoying a renaissance. This popularity is driven in part by the carbon-neutral nature of most biomass (i.e., no net CO2 emissions). The rising cost of fossil fuels and incentives for switching drive consumer decisions toward renewable energy. Also, renewable-energy portfolio mandates require that utilities construct renewable power plants.
One option is converting existing coal-fired power plants into plants that can utilize biomass, or be co-fired with biomass. The co-firing is limited in part because of undesired changes in the resulting ash composition, such as high quantities of alkali metals. Biomass pellets are also increasingly used in uncontrolled domestic heat generators. European Committee for Standardization (www.cen.eu), Technical Committee guidelines for domestic heating pellets recommends ash content less than 0.7%, limiting materials that can be used.
One challenge to combusting biomass is its high moisture content. Living and freshly cut biomass typically contains moisture between 40% and 60%. In loose storage, the biomass dryness can reach air-dry moisture of about 10%. This drying of wood is slow, typically requiring at least a full summer season. This necessitates double handling and increases procurement cost.
It can be advantageous to first pelletize biomass, to increase energy efficiencies of boilers. Pelletizing processes can drive moisture out of the biomass, by using part of the biomass energy, waste heat, or a fossil fuel. The final moisture from pelletizing is typically 5-7%, which is similar to moisture of coal. Boiler efficiencies increase approximately half a percent with each percentage removal of moisture.
In biomass, cellulose and hemicellulose each have about half of the calorific heat value of coal, because of high oxygen content of polymeric sugar constituents. Lignin has a similar calorific heat value to coal, but sulfur is nearly absent. The combined energy content of biomass is typically 8,000-9,000 Btu/lb, as compared to 10,000-14,000 Btu/lb in coal. Because of high oxygen content and moisture in biomass, the boiler efficiency for biomass firing typically ranges from 50-65%. A large portion of heat generated in combustion escapes as steam through the stack. Therefore, converting coal-burning boilers to biomass firing may reduce boiler capacity by as much as 60%.
Feeding irregularly shaped biomass also represents a challenge. Pelletizing can produce uniformly sized material that does not bridge or lodge easily in a storage silo. On the other hand, the pelletized material can absorb moisture, if stored loosely outdoors.
Another obstacle is presented by the ash in the biomass. Hardwood and softwood stem and forest trimmings contain 0.4% to 0.8% ash that is rich in calcium and potassium. Other biomass materials including pulp and paper sludge, paper waste, recycled paper and construction waste, can contain up to 30% ash. Such ash includes minerals in plant capillaries, dirt on the surface, and coating in the paper. The wood exposed to salt water contains elevated levels of sodium and chlorides.
Agricultural residues of annual plants, such as corn stover, corn fiber, wheat straw, sugarcane bagasse, rice straw, oat straw, barley straw, miscanthus, and energy cane can contain up to 10% or more ash that is rich in silica, potassium, and chlorine. The agricultural residue material is very lean in sulfur, typically less than 0.1%, versus coal sulfur content of 0.5-7.5%. Significant minerals in these annual agricultural residues include potassium, sodium, silica, calcium, and corrosive halogens such as chlorides.
Upon combustion at high temperatures, metals and halogens volatilize to aerosols and carry over from the boiler with flue gas. The cooling of fly ash creates microscopic particles that are found to cause respiratory illnesses. Flue-gas treatment for particulate removal includes cyclones, scrubbers, and electrostatic precipitators (ESP). These environmental controls in the central power plant are expensive and, in domestic applications, tend to be cost-prohibitive. Recent Maximum Achievable Control Technology (MACT) legislation by the U.S. EPA seeks to control particulate emissions from large biomass power plants. Other minerals such as calcium and silica remain in the bottom of the boiler and have tendency to form clinkers and to scale (slag) in the boiler tubes. Alkaline chloride salts can cause corrosion of the boiler tubes.
What are needed are processes and apparatus to prepare biomass, including wood and agricultural residues, into clean, low-ash biomass for improved combustion, with or without first pelletizing the biomass. The low-ash biomass should be capable of being fired alone or in combination with another solid fuel. It would be desirable for these processes to also have good potential to recover various co-products, such as fermentable sugars, fertilizers, and lignin.